Hydro-pneumatic energy storage systems usually include at least one hydro-pneumatic accumulator, the accumulator comprising a rigid hollow vessel that encloses a storage space. Normally, the storage space is divided into a closed compressible volume filled with an amount of a process gas such as nitrogen and into a variable space for receiving a non-compressible hydraulic fluid such as oil. Typically, the variable space is in fluid communication with an exterior of the vessel through one or more valves.
Energy may be stored in the system by discharging a quantity of a non-compressible hydraulic fluid into the variable space inside the vessel such that the process gas contained in the closed volume inside the vessel is compressed and pressurized. The gas may then be kept in the pressurized state by closing the valve. When needed, the hydrostatic energy stored in the pressurized gas may be released by opening the valve and by letting the gas expand, the expanding gas thereby displacing the non-compressible hydraulic fluid stored within the vessel out of the vessel, thus creating a fluid flow. The flow of hydraulic fluid generated by the expanding gas may then drive a hydraulic device such as a hydraulic motor or a hydraulic piston, for example.
It is known that the amount of energy that may be stored in a hydro-pneumatic energy-storage system may be increased by increasing the system size and/or by increasing the precharge pressure of the process gas. However, space for accommodating the accumulators of the storage system is often limited. Furthermore, the precharge pressure of the process gas usually has to more or less match the operating pressure of the hydraulic circuit to which the storage system is intended to be connected.